Asymmetric and composite membranes capable of selectively separating one component of a gaseous mixture from another component are known in the art. For practical commercial operations the composite membranes must be durable, essentially free of imperfections, capable of achieving an acceptable level of selectivity of the desired component and exhibit high permeation rate for the fast permeating component of the gas mixture. Often, however, the gas separation layer deposited on the surface of the substrate of a substrate does not fully meet the required needs; for instance, it may not be adequately resistant to the solvent effects of the gas/vapor mixture that can condense onto the membrane surface during the gas separation process or it may contain microscopic residual pores or other defects. Thus, continued efforts are being expended to improve the structure and efficiency of composite membranes.
Integrally skinned asymmetric membranes are currently used extensively for numerous gas separation processes. Though manufacturing of essentially defect-free ultrahigh flux asymmetric membrane is known in the art, see for example U.S. Pat. No. 4,902,422 and U.S. Pat. No. 4,772,392, it is known to be excessively difficult. Thus, it is common in the art to subject gas separation membranes to treatments that effectively eliminate defects which may be present in ultrathin membrane separation layers. Henis and Tripodi in U.S. Pat. No. 4,230,463 have addressed the presence of defects in asymmetric gas separation membranes by applying a coating. The multi-component membranes produced by this coating process typically comprise a silicone rubber coating on the surface of an asymmetric membrane made of a glassy polymer. Additional defect-repair methods can be found in U.S. Pat. Nos. 4,877,528, 4,746,333 and 4,776,936.
A different class of gas separation membranes, the composite membranes, is produced by depositing a thin gas separation layer on a porous support wherein the material of the deposited layer determines the gas separation characteristics of the overall structure. These composite membranes are sometimes more advantageous since they allow decoupling of the material requirements for a particular gas separation application from engineering design requirements of the porous support. A variety of separation layer materials, support structures and membrane manufacturing methods are known in the art for producing composite membranes. Typical examples of composite gas separation membranes can be found in U.S. Pat. Nos. 4,243,701, 3,980,456, 4,602,911 and 4,881,954.
Difficulties are also sometimes encountered in the preparation of defect-free composite membranes for gas separation, particularly for gas separation materials with very high cohesive energy density such as polymeric materials that contain ionic groups. We have discovered that certain deficiencies of such composite membranes can be corrected by treating the gas separation layer per se with a dilute solution of a material that will chemically react or ionically bond to the material of the gas separation layer. In addition to improved gas separation performance, the membranes of this invention frequently will exhibit improved solvent resistance characteristics.
U.S. Pat. No. 3,980,456, issued Sept. 14, 1976 to W. R. Browall, discloses a process for patching breaches in a composite membrane by coating the entire outer surface of the composite membrane by casting a layer of sealing polymer material over the entire surface (column 2, lines 37 to 40) of the composite membrane so as to cover surfaces particle impurities and seal pinholes. U.S. Pat. No. 4,767,422, issued Aug. 30, 1988 to B. Bikson, et al., discloses a method for repairing defects in composite membranes by post-treating with a volatile solvent, with or without minute amounts of additives, followed by evaporation. The concept of treating the gas separation layer of a composite membrane with a reactive treating agent and ionically binding a reactive treating agent to the gas separation layer of the composite membrane is neither suggested nor disclosed in U.S. Pat. No. 3,980,456 and U.S. Pat. No. 4,767,422.
U.S. Pat. No. 4,602,922, issued July 29, 1986, I. Cabasso, et al., discloses the preparation of improved composite membranes by depositing a thin layer of aminoorganofunctional polysiloxane on the surface of a highly porous polymer substrate, such as polysulfone substrate, and in-situ crosslinking the amino siloxane units with diisocyanate and using the crosslinked polysiloxane as a gutter layer. A gas separation layer is coated on the gutter layer to provide a double-layer composite membrane which has a higher separation factor than the crosslinked polysiloxanes.
U.S. Pat. No. 4,243,701, issued Jan. 6, 1981 to Riley, et al., discloses a method for coating a preformed porous support membrane by passing a surface thereof through a solution which contains a mixture of the semipermeable gas separation membrane forming prepolymer and a crosslinking agent and then heating the coated surface to crosslink the prepolymer to form the composite membrane. It does not disclose or suggest treating a preformed composite membrane with a reactive treating agent.
In U.S. Pat. No. 4,877,528, issued Oct. 31, 1989 to D. T. Friesen, et al., siloxane-grafted cellulosic semipermeable membranes are disclosed. The defects in the gas separation layer of an asymmetric cellulosic gas separation membrane are sealed by covalently binding polysiloxane containing functional groups to the cellulosic material. The two reactive materials react and chemically bind by ether, ester, amide or acrylate linkages to form a siloxane-grafted cellulosic membrane having improved selectivity.
In U.S. Pat. No. 4,863,496, issued Sept. 5, 1989 to O. M. Ekiner, et al., reactive post-treatments for gas separation membranes are disclosed. Specifically the process disclosed entails applying a reactive monomeric material to the surface and polymerizing the monomeric material applied on the surface of the gas separation membrane in order to improve the permselectivity of the membrane with at least one pair of gases.
U.S. Pat. No. 4,634,531, issued Jan. 6, 1987 to Y. Nakagawa, et. al., relates to the sequential treatment of semipermeable membranes with at least two different water soluble organic materials that will react and form a water insoluble or very slightly water soluble material on the semipermeable membrane. The invention requires a combination of at least two sequential treatments with aqueous solutions of materials which mutually react at once upon contact, for example, a first treatment with an aqueous amine solution followed by a second treatment with an aqueous aldehyde solution. This series of sequential treatments is applied to the surface of an asymmetric membrane, or to the surface of a composite membrane composed of a support having a thin film barrier coated thereon. The treatment effected by this invention serves to form an additional layer on the surfaces of the asymmetric membranes and the composite membranes treated. The patent teaches the formation of an additional layer on the surface of the composite membrane, it does not suggest or disclose the concept of treating the gas separation layer per se of the composite membrane with a treating agent that will react with the gas separation layer or ionically bond to the gas separation layer per se of the composite membrane.